Acoustic sensors are known and used, for example, in apparatuses for monitoring the machining process of computer numerical control (“CNC”) machine tools, as lathes, grinding machines, milling machines, etc. such apparatuses are able to detect, by means of sensors, the magnitude of physical features connected to the process to be checked, for example the wear of the tool, and indicate to the machine numerical control, or directly to the operator, the need to perform maintenance and/or corrective procedures.
An apparatus of this kind is disclosed in International patent application No. PCT/EP02/07519, filed by the same applicants of the present patent application.
In machining process monitorings on grinding machines, the acoustic sensors are generally coupled to the flange for securing to the spindle the grinding wheel (as shown in simplified form in FIG. 2) or the dressing wheel and enable to detect vibrations generated by the occurrence of contact between the grinding wheel (or the dressing wheel) and the workpiece (or the grinding wheel), in the course of the machining.
More specifically, the acoustic sensors presently utilized in apparatuses of the type described in the formerly mentioned patent application include a rotor with a casing coupled to a movable part as, for example, the grinding wheel of a grinding machine. Furthermore, the rotor includes a piezoelectric transducer glued to the casing and electric circuits for conditioning the output signal of the piezoelectric transducer. The rotor is coupled, by means of a transformer type coupling, to a stator connected, for example, to the bed of the grinding machine, that includes further electric circuits for processing and transmitting the signal received from the electric circuits of the rotor.
An acoustic sensor of this type is disclosed in European patent application EP-A-0446849.
The structure and the dimensions of the components of an acoustic sensor as the one disclosed in the formerly mentioned European patent application, provide good standards of performance in response to acoustic signals with frequencies reaching up to a few hundreds of KHz.
In a grinding machine in which it is desired to reach high rotation speeds, the grinding wheel (and/or the dressing wheel) is coupled to an electrospindle. The use of high speed electrospindles generates an acoustic background noise that, in the frequency range utilized by the known acoustic sensors (50–250 KHz), is added to the acoustic signal generated by the occurrence of contact between the grinding wheel and the workpiece or the dressing wheel thus making the detecting of said contact extremely difficult. In the graph of FIG. 1 there are shown, in logarithmic scale and as a function of frequency F, the trends of the spectral densities DS1 and DS2 of the background noise generated, in a grinding machine, by a traditional type spindle and by a high speed electrospindle, respectively, carrying the grinding wheel or the dressing wheel.
Furthermore, the dashed line in FIG. 1 indicates the trend of the spectral density DS3 of the acoustic signal generated by contact occurring between grinding wheel and workpiece or dressing wheel (that is the signal to be detected), that does not depend on the type of operation of the grinding wheel and, within certain limits, on the rotation speed of the grinding wheel itself. The graph of FIG. 1 shows how, at high speed (DS2), the ratio between the signal generated by the occurrence of contact between grinding wheel and workpiece and background noise is critical in the range of frequencies 50–250 KHz. As a consequence, the checking of the machining process, performed by an acoustic sensor operating in said range of frequencies, is extremely problematic. The ratio between useful signal and background noise is acceptable at higher frequencies (in the range 500 KHz–1 MHz). Thus, in order to perform machining process checkings on grinding machines with a grinding wheel rotating at high speed, activated by an electrospindle, the acoustic sensors like the one disclosed in patent application No. EP-A-0446849 have poor performances in consideration of the limited range of frequencies within which there is guaranteed good response.